1. Field of the Invention
This invention relates generally to the field of differential amplifiers. More particularly, this invention relates to a differential amplifier circuit and has variable gain.
2. Background of the Invention
Professional audio equipment frequently utilizes differential line input stages. Differential amplifiers used in these applications frequently require a wide range (e.g. 30 db) of variable gain with an acceptably low input impedance (for good noise performance) and an acceptably high value of common mode rejection (e.g. &gt;35 db). FIG. 1 illustrates a conventional design for such an amplifier stage.
In FIG. 1, input signals V.sub.A and V.sub.B are applied to resistors 10 and 12 respectively which are connected to the inverting and non-inverting inputs of an operational amplifier 16. A fixed amount of differential gain is provided according to the values of resistors 10 and 12 in cooperation with resistor 20 and feedback resistor 22 in a known manner. The fixed input stage of amplification provided by amplifier 16 permits the designer to design the circuit for suitably low impedance and high common mode rejection. The output of amplifier 16 is provided to a single ended gain stage through a resistive divider made up of resistors 26 and 30 to the non-inverting input of a second amplifier 34. Negative feedback is provided through resistor 38 to the inverting input of amplifier 34. The gain of amplifier 34 is adjustable via a potentiometer 40 which is in series with a resistor 42 and connected between the inverting input of amplifier 34 and ground. The value of resistor 42 limits the maximum gain of amplifier 34 to a predetermined value. The sum of resistor 42 and the full value of potentiometer 40 establish the minimum gain of amplifier 34.
The circuit of FIG. 1, and similar circuits which use multiple operational amplifiers (often three or more are used) provide very high performance and maximum control of all parameters associated with the differential amplifier. However, it is often desirable to reduce the number of amplifier stages and associated components in order to increase reliability, reduce size and reduce cost. Several circuits have been proposed to accomplish this purpose. The circuit of FIG. 2, for example, uses a single operational amplifier 50 as an input stage and variable gain stage. Voltage inputs V.sub.A and V.sub.B are provided to resistors 60 and 64 which are respectively coupled to the inverting and non-inverting inputs of amplifier 50. The gain of this arrangement is established by a network of 5 resistors. 66, 68, 70, 72 and 74 along with a potentiometer 78. Negative feedback is ultimately provided through resistor 66 while positive feedback is provided through resistor 72. Due to the presence of positive feedback, the circuit of FIG. 2 has the potential of instability which cannot be tolerated in some high performance audio applications.
Another single amplifier arrangement is shown in FIG. 3 with V.sub.A and V.sub.B again provided through resistors 80 and 82 to the inverting and non-inverting inputs of amplifier 86. Negative feedback is provided through a pair of feedback resistors 90 and 92 with a potentiometer 94 in series with a resistor 96 connecting the junction of resistors 90 and 92 to ground. Resistor 98 connects between the non-inverting input and ground. This circuit meets many of the needs outlined above except that the input impedance of the circuit must be relatively high in order to achieve a minimum gain value in the range of -10 db.
Another common circuit for providing a single amplifier variable differential amplifier is shown in FIG. 4. In this circuit signals V.sub.A and V.sub.B are provided through resistors 102 and 104 to the inverting and non-inverting input terminals of amplifier 108. The non-inverting terminal is also connected through resistor 110 to ground. A potentiometer 114 is connected from output to inverting input to provide variable negative feedback. While this circuit is simple in design and has a stable input impedance, the common mode rejection ratio (CMRR) varies widely with the gain selected by adjustment of potentiometer 114. In many cases the CMRR can become extremely low (e.g. less than 10 db) which can contribute to audible noise in an audio environment.
Accordingly, it is desirable to provide a differential amplifier which overcomes the shortcomings of the circuits of FIGS. 1 through 4.